1. Field of the Invention
This invention relates generally to the fabrication of a giant magnetoresistive (GMR) magnetic field sensor in the current-perpendicular-to-plane (CPP) configuration, more specifically to a method of improving photolithographic masks used in the patterning of such devices.
2. Description of the Related Art
Magnetic read sensors that utilize the giant magnetoresistive (GMR) effect for their operation must be patterned to produce a required trackwidth. Such patterning is conventionally done using a single photolithographic lift-off mask as both an etching stencil and a deposition mask. The shape of the stencil portion of such a mask permits the necessary trimming of the deposited layers to the required trackwidth and then the mask is used to allow deposition of additional layers (eg. conduction lead layers, biasing layers and/or insulation layers) within the removed regions.
In current head fabrication technology, the track width and back edge of a read sensor, such as a CPP read sensor, are defined by two successive photolithographic processes. Referring to FIG. 1a, there is shown an overhead view of a first lithographic mask (10), which has a kind of “dog-bone” shape, formed on the surface of a sensor stack. When the all portions of the stack extending beyond the periphery of the mask are removed by etching, the removed portions are typically refilled with some other material (eg. dielectric material). The patterned (15) and refilled (17) configuration is now shown in FIG. 1b, where the narrow central portion (circled (30)) defines the track width of the device. At this point a second mask (20), substantially identical to the first but oriented orthogonally to the first mask, is formed over the configuration. When an etch and refill is performed using this mask, the final definition of the device is obtained, both in terms of its track width and its back edge. Unfortunately, this double masking operation becomes impractical as the dimensions of the device fall below 100 nm. As is shown in FIG. 1c, the procedure produces uneven depths in the deposited dielectric (or other) materials in the areas indicated as 1, 2 and 3. This, in turn, can result in shorting to the overlaid top lead layer.
One way to overcome this problem is to combine the two step process into a single step by the use of a mask shaped as shown in FIG. 2a. Unfortunately, due to insufficient photoexposure dosage at the corners (50) of such a mask shape, the sharp corners become poorly defined and the mask is actually produced with the rounded corners (55) shown in FIG. 2b. 
To avoid such rounded corners, one can apply enhanced photoexposure to the corner areas, as shown by the outlined regions (15) in FIG. 3a. When the photoresist forming the upper portion of the mask is developed, however, the overexposed regions are much more resistant to dissolution than the layer of material (eg. PMGI (polydimethylglutarimide)) beneath the photoresist, and that overexposed portion of the mask becomes suspended, with nothing beneath it (region (25) in FIG. 3b). To avoid this problem, a method has been developed which allows the dissolution of the mask underlayer to be controlled, so that the sharp corner and edge details can be defined and developed and be supported by an underlayer.
It is noted that the inability to control the dissolution of a bi-layer mask underlayer causes significant problems not only, as in the present case, when the mask has sharp edges and corners, but also when the mask has a thin central portion. That problem has been addressed Han et al. (U.S. Pat. No. 6,493,926), which is assigned to the same assignee as the present invention and which is fully incorporated herein by reference. Han discusses several problems associated with prior art lift-off masks in which an upper (stencil) layer of photoresist is formed over a lower, undercut, pedestal, layer. In such mask designs the width of the pedestal layer becomes a critical factor in the proper performance of the mask during the deposition stage. If the pedestal is undercut too much, the upper portion of the mask can collapse prematurely under the weight of deposition residue making a clean lift-off of the mask impossible. On the other hand, if the pedestal is insufficiently undercut, subsequent depositions can build up against the pedestal, called “fencing,” leading to excessive thicknesses of the deposited material and short-circuiting of deposited conductive layers. To overcome the difficulties of forming properly and consistently undercut pedestals and for use in forming trackwidths of approximately 0.5 microns, Han et al. teach the formation of a bi-layer suspension-bridge mask formation, in which there is no pedestal directly beneath the upper portion of the mask, but wherein the upper portion is supported on two pedestals that are laterally disposed beneath two distal ends of the mask. The complete elimination of any support directly beneath the mask thereby avoids the problems associated with insufficient or overly-sufficient pedestal undercut. The formation taught by Han et al. requires that the portion of the mask that would ordinarily be beneath the upper portion be completely removed, so that the upper portion is suspended above the device to be patterned and does not contact it. This object is achieved by forming the pedestal portion of the mask of a layer of PMGI, while forming the upper portion of the mask of a layer of photoresist material. Application of a proper developing solution thereupon dissolves the lower PMGI portion preferentially relative to the photoresist upper portion, removing the PMGI except beneath the end portions where it remains to serve as a support.
Whereas the method of Han et al. eliminates the problem of inadequately controlled PMGI dissolution by essentially completely dissolving the PMGI, to form a completely suspended region, such complete suspension in the present case exacerbates the problem. The present inventors, in related Application HTIRC-03-011, which is fully incorporated herein by reference, have applied a novel ozone-assisted method of developing a mask of the type discussed by Han, so that a thin central region of the mask can be formed with a thin undercut PMGI region rather than having to remove the PMGI underlayer completely. While the present invention relates to a differently shaped mask, with development problems unlike those associated with the formation of a narrow central mask region, ozone can still be applied advantageously to control underlayer dissolution. It is the purpose of the present invention, therefore, to teach a method of forming a bilayer lift-off mask having sharply defined edge and corner regions, wherein the method includes the use of ozone to assist in the controlled dissolution of a mask underlayer.